This invention relates to motor protectors and more particularly to a device which can be used to control the energization of a small, low current, intermittent duty motor such as that used to operate door locks in automobiles. It is conventional in such applications to use very small motors, typically an inch or less in diameter by less than two inches in length, twelve volt, direct current, permanent magnet, and electrically reversible. These motors are designed to move an actuator of given weight, e.g. 2.27 Kg, a certain distance within a selected time period. When the actuator moves to its extremity in either direction the rotor will be locked. If the manually actuated switch is still energized, as by a door lock button which sticks, the motor is then deenergized by some type of current interrupting device. Current interrupting devices comprising a bimetallic member which moves to separate electrical contacts to open the motor circuit are known but have not been fully satisfactory when used with these small motors. Such devices are relatively expensive and have limited service life since they have moving parts. Further it has been found to be virtually impossible to adequately protect the motor due to the limited snap action motion as a result of the constraints of space. As a result, nuisance trips are likely to occur due to the contacts creeping open. This causes fast cycling under locked rotor conditions and ultimately untimely motor failure. Another problem occurs when calibration of the device is adversely affected by shocks such as those produced by slamming of the door over the life of the device.
A solid state PTC protector, since it has no moving parts, is not subject to the above noted deficiencies however due to the small size of the motor as well as the small volume of space available for such a device none have heretofore been available for this purpose. A typical motor of this type is rated as 0.005 horsepower or 0.4 watts. Conventional PTC devices are too massive to provide the required cycle timing, i.e., it takes too long for a conventional PTC device to reach its reset temperature once it goes above its anomaly temperature, or it has too high a base resistance for the motor with which it is to be serially connected as in the case of ceramic PTC, or it has too low an anomaly temperature for the elevated temperature in the automotive environment as in the case of polymer PTC, i.e., the PTC would reset too quickly.
It is therefore an object of the invention to provide a motor protector which is particularly well suited for use with low current motors, in the order of 8 amps, 12 volt DC, which in normal operation runs in one direction until an actuator moved by the motor reaches a limit and a manually actuated switch is deactuated. The motor protector's function is to deenergize the motor when it goes into the locked rotor condition before the motor is damaged from overheating. Then, after the motor protector cools sufficiently to reset, the motor can be energized to move the actuator in the opposite direction until another limit is reached. The motor goes into locked rotor condition in every cycle but normal switch operation should allow use of a door lock without operation of the protector. However, if the button sticks or is held in an actuated position for an excessively long period of time, the motor will overheat due to the locked rotor so that the selection of the motor protector is very critical particularly with regard to matching the protector's characteristics to the temperature characteristic of the motor. It is an object of the invention to provide such a protector which can be readily manufactured using efficient, mass production techniques. Yet another object is the provision of a protector which is inexpensive yet has a long life expectancy.
Other details, uses and advantages of the invention will be readily apparent from the exemplary embodiment thereof presented in the following specification, claims and drawings.
In accordance with the invention the objects are realized by mounting two wafers of ceramic PTC material approximately 0.33-0.36 mm in thickness, each having a base resistance of approximately 0.6 ohms, and connecting them in parallel circuit relationship in a package which has minimal mass. The package comprises a housing formed out of suitable material such as a sheet of cold rolled steel approximately 0.36 to 0.40 mm in thickness formed into top and bottom portions each having a recess and with the two portions connected together but hinged so that the sheet can be bent back on itself with the recesses in aligned facing relationship with one another forming a switch cavity. The housing is held so that one recess faces upwardly, the other portion is bent upwardly approximately 90.degree., a first PTC wafer is placed in the upwardly facing recess, a center spring contact plate having a plurality of wafer contacting surfaces struck therefrom on two opposite faces of the plate is placed on top of the wafer and a second PTC wafer is placed on top of the center contact plate. Apertures are provided in the side walls of the housing extending into the top and bottom walls to allow access of positioning fingers which locate the wafers and center contact as desired relative to the housing. The portion of the housing that had previously been bent upwardly is then bent all the way over until welding tabs extending from that portion come into contact with welding projections formed on a ledge extending from the first portion and the housing is welded together. Protrusions are formed in the housing and the spring contact plate to engage the PTC wafers and are so located that three protrusions in the housing are in alignment with three protrusions on the spring contact plate for each wafer so that no torsional forces are imparted to the wafers while at the same time effective electrical contact is achieved.
Placement of the center contact plate between the two portions of the housing results in spacing the two portions from one another along the sides of the housing to provide extra ventilation as well as making the fitting of the two portions together less critical compared to conventional two part housings which fit closely together. The center contact plate is provided with strengthening ribs which are disposed at such an angle that they, along with angled side walls of the housing, prevent shorting across the wafer even in the event that a wafer moves into physical contact with a rib or side wall.
Three contact fingers struck from the center plate are bent from the plane of the plate in one direction a selected distance while a pair of contact fingers, also struck from the center plate, are bent from the plane of the plate in the opposite direction a lesser distance and cooperate with a protrusion formed in the body of the center plate to make electrical contact with the wafers.
A pair of gaskets are placed over an end of the center contact plate with one gasket having a preformed depressed area surrounding apertures which depressed areas fit into mating apertures in the center contact plate and over the weld projections on the housing ledge so that any flash occasioned by the welding operation will not form a bridge between the housing and the center contact plate.